Question

I want to create hashtable which would take each String from my array and assign it to unique integer value. My array is read from file and assigned to array like this:

public void readFile() throws Exception{

    FileInputStream in = new FileInputStream("words.txt");
    BufferedReader br = new BufferedReader(new InputStreamReader(in));

    String strLine;
    wordsList = new String[getNumberOfLines()];   

    for (int j = 0; j < wordsList.length; j++){
    wordsList[j] = br.readLine();

    }
    in.close();
}

Using this array I wrote method to create hash table like this:

String currentWord;
private Hashtable <String,Integer> wordsHashTable;
LinesReader lr = new LinesReader();
int i;
String[] listOfWords;

public boolean insertValues() throws Exception{
    for (i=0; i<lr.getNumberOfLines();i++){
        lr.readFile();
        listOfWords = lr.returnsWordList();
        currentWord = listOfWords[i];
        wordsHashTable.put(currentWord, i+1);
    }
    return wordsHashTable.isEmpty(); //testing purposes only
}

It throws NullPointer exception at line: wordsHashTable.put(currentWord, i+1);
Any ideas where I messed up?

Answer 1

You must initialize your wordsHashTable with an instance of the class:

private Hashtable <String,Integer> wordsHashTable = new Hashtable<>();

However, do note that the Hashtable class is obsolete; you should use java.util.HashMap instead.